The following discussion providing some background to the invention is intended to facilitate a better understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
It is known that plastic pipes can be made by helically winding a plastic strip having a series of spaced apart upstanding ribs extending longitudinally of the strip, either at room temperature or at an elevated temperature where the plastic becomes more flexible. This form of helically wound tube is already known in the piping industry and is described in Patents by the applicant relating both to the form of the plastic strip, the form of the tube and the form of the machine by means of which the pipes or tubes are produced from such strips.
For these pipes to perform in a high performance applications, in order to attain the necessary degree of strength, the wall thickness of the plastic strip must be quite substantial, as well as that of the ribs. Alternatively the finished pipes or tubes can be reinforced with strengthening or reinforcing members.
In applications where the reinforced tubes or pipes are buried in a trench or subjected to high earth loads, the strength of the pipe or tube is of extreme importance.
The applicant's Australian Patent No. AU607431 discloses a method of producing a reinforced plastic tube utilising a reinforcing member placed between the ribs in such a manner that the deflection resistance of the finished pipe or tube is materially increased. The reinforcing member comprises a metal member having a profile of U-shape cross-section, the free ends of the reinforcing member being designed to engage beneath opposed flange formations of a pair of adjacent ribs to thereby lock the metal strip in position between the ribs and in turn stiffen the ribs and the finished pipe.
The applicant's Australian Patent No. AU661047 discloses an improvement over the disclosure of Australian Patent No. AU607431 referred to above. The improvement is provided by the provision of a reinforcing member having a central body portion of inverted U or V-shaped cross-section that has a radial height greater than the height of the ribs whereby the effective external diameter of the composite pipe is substantially increased. This provides a stiffer pipe.
The applicant's Australian Patent No. AU2003227090 discloses a further improvement over the disclosure of Australian Patent No. AU661047 referred to above. The improvement is provided by the provision of a reinforcing member having height to thickness ratio of at least 3:1 and orientated substantially perpendicular to the base of the strip. The inner face of the strip forms a continuous surface below the reinforcing strip. The provision of the aforementioned reinforcing strip reinforces the pipe against radial crushing loads in a more efficient way than prior art reinforcement, while the inner face provides a smooth internal pipe surface and separates the reinforcing strip from the fluid within the pipe.
Prior to commercialisation of applicant's Australian Patent No. AU2003227090, helically wound composite pipes were formed in a multi-stage operation. The plastic body was extruded and then was helically wound to form a pipe. Elongate steel reinforcing members were separately roll-formed into a profile providing the required stiffness (such as the inverted U or V-shaped profiles referred to above). The roll formed steel profile was then rolled to a radius approximating that of the helically wound plastics body. Finally, the profiled and radiused reinforcing member or members were wound on to the outside of the plastics pipe to form a composite pipe of the requisite stiffness.
When using the reinforcing members disclosed in Australian Patent Nos AU607431 and AU661047, the step of rolling the steel reinforcing member to a radius of approximating that of the plastics pipe involved straining the steel reinforcing member beyond its elastic limit. This required the application of considerable force during the rolling process. In contrast, winding of the extruded plastics profile into a helical pipe generally requires much less force due to the material properties of the plastics. Using the composite profile of the applicant's Australian Patent No. AU2003227090 it became possible to wind a preformed steel and plastic composite strip directly into a pipe from a spool of composite profile.
An object of the present invention is to provide certain improvements, beyond those disclosed in the aforesaid Patent Specifications AU607431, AU661047 and AU2003227090, to strips windable into reinforced helically wound plastics tube or pipe and to pipes wound from such strips.